This invention relates to charge-injection-devices (CID), based on, e.g., indium antimonide, used in self-scanned infrared image detectors and more particularly to such a device in which blooming at low input levels is reduced without loss of sensitivity by shielding the y-electrodes of each detector. Heretofore multi-element detectors have been formed wherein each detector element had one contact and one lead connected to each detector. Such devices required many electrically conductive wires outside the CID, which are cumbersome, use a lot of space, and add weight to the system. Most of the electrical wires have been eliminated in a two-dimensional single-column structure in which the output of each detector is connected to a common conductor and the detector elements are scanned by a shift register to read out each detector in order.
Presently two-dimensional charge-injection devices have been made into an array having, for example, sixteen by twenty-four detector elements. Each detector element has two contacts, each of which has a lead that connects with a shared conductor. The number of conductors is comparable only to the number of detector rows and columns. For instance, the detector array with 16.times.24 elements will have 16 row leads and 24 column leads with the x-electrode of each detector element connected to a row lead and the y-electrode of each detector element connected to a column lead. The column leads are connected to a common conductor which is connected to a signal preamplifier followed by appropriate electronic circuitry and a suitable display or automatic recognition device. The x-electrode of each element of each row is gated by a row shift-register and the outputs to the column leads for each column of elements are gated by a column shift-register.
The prior art array is formed by 16.times.24 elements each of which has both of its contact areas exposed to the image excitation radiation. Infrared radiation generates hole-electron pairs in the substrate and minority carriers are collected in potential wells, one associated with each electrode. When the charge in one y-electrode is read out, all the rest of the y-electrodes in that column are also read out, because they are connected together electrically to a common conductor.
In order that signal-charge be read out at a time identifiable by the external circuitry with its image element of origin, distinguishable in particular from other image elements belonging to the same column, the signal charge must be stored at the x-electrode, and then transferred to the y-electrode for read-out by application of suitable voltage pulses to the x- and y-electrode leads, at the proper time for read-out. For example, if the signal charge is initially integrated at a y-electrode, it must be transferred to the x-electrode for storage, and then back to the y-electrode for read-out, becuase the y-electrodes are the ones connected to the output circuitry. It has been determined that in an array such as set forth above only about 20% of the charge is transferred from the x-electrode to the y-electrode or vice versa, and the other 80% is lost. High transfer efficiency is one of the most difficult properties of a CID to achieve in fabrication. The state-of-the-art of other types of CID's for infrared imaging is believed to be similarly poor. The resultant efficiency of two successive transfers is so low that signal charge initially integrated at a y-electrode contributes almost nothing to the useful image. That signal charge can not ordinarily be identified by the external circuits with the image element of its origin, but only spuriously with some other image element of the same column, because of blooming. Thus, the signal charge integrated at a y-electrode manifests itself as a spurious localized output, in case of a very short pulse of illumination, or a blooming of the whole column, in case of steady illumination. In summary, integration at the y-electrodes does not contribute to a faithful image, but is a deleterious source of blooming.
The operation of such a two-dimensional CID requires many transfers of signal charge back and forth between the two electrodes of each image element, before the signal charge is finally read out. Since all of the x-electrodes in any row of imaging elements are connected to the same row conductor, it follows that a voltage pulse applied to a row conductor for the purpose of transferring charge from one x-electrode to its companion y-electrode will also have that effect in the other image elements of the row. Therefore, whenever charge is transferred from the x- to y-electrode of any image element, the other image element in the same row are similarly transferred, thereby losing most of their integrated signal charge. It follows that the image elements effectively integrate signal charge only for a time equal to the time between read-outs of the same row in successive columns. The limitation of the integration time to this relatively short time, rather than to the relatively long time required to read out all image elements, prevents a steady input from being integrated for a long time at the x-electrode until it produces a large output which could predominate over the blooming associated with integration at the y-electrode.